1. Field
This disclosure relates to a method of cleaning well bores, and in particular cleaning casings of oil wells, geothermal energy wells, and other wells.
2. Description of the Related Art
Petroleum products such as oil and natural gas are commonly produced by drilling a borehole or wellbore into the earth through the oil or gas producing subsurface formation. In some cases, the petroleum product may be extracted directly through the drilled borehole. More commonly, a pipe casing is placed in the borehole, for example to prevent collapse of the borehole or to prevent contamination of other subsurface formations. In the oil production industry, a distinction is commonly made between a “casing” (a pipe string that extends to the top of the borehole) and a “liner” (a pipe string, hung within a larger casing, that does not extend to the top of the borehole). This distinction is not relevant in this patent, and the term “casing” as used herein refers to any pipe string within a borehole. The annular space between the outside of the casing and the borehole may be filled, in whole or in part, with cement to retain the casing in position and to prevent fluids from traveling between subsurface layers via the annular space. An appropriate portion of the casing may be perforated to allow the petroleum product to flow from the producing formation into the casing. In some wells, the annular space between the outside of the perforated portion of the casing and the borehole may be filled with gravel. In this patent, the combination of the borehole, the casing, the cement, the gravel if present, and any associated surface equipment will be referred to generally as a “well”. Similar wells may be used to extract superheated water for geothermal power generation or to inject water or other fluids into a subsurface formation to simulate oil or gas production.
After a period of production of fluids from a well or injection of fluids into a well, the perforations or openings in the casing may become plugged or encrusted, restricting the flow of fluids into or out of the casing. Materials that may be deposited in the casing include paraffin, asphalt, other petroleum products, mineral scale, and biological organisms. Unchecked, such deposits may reduce the flow of fluids until the well is not useful for its intended purpose, necessitating re-perforating or replacing portions of the casing.
A number of approaches have been suggested for cleaning flow-restricting deposits from wells. These approaches include treatment with acids or other chemicals, ultrasonic vibrations, or mechanical shock waves resulting from, for example, detonation of gases or explosives within the well bore. Such well cleaning techniques have limited effect and/or risk erosion of or other damage to the well casing.
Another proposed technique for cleaning wells is to use repetitive electrical discharges to produce mechanical shock waves. Electric discharge devices, commonly called “sparkers”, have been used to generate acoustic waves for subsea surface mapping. Such devices create a shock wave by discharging stored energy between a pair of electrodes immersed in the body of water being mapped.
U.S. Pat. No. 4,343,356 describes a high energy electric discharge device designed to be lowered into a well casing. The device is discharged at intervals as it is lowered into the well casing to create shock waves to clean the adjacent portions of the casing. Each electrical discharge may also generate ultraviolet light and/or ozone, which may also contribute to cleaning the adjacent portions of the casing of organic or biological materials.
U.S. Pat. No. 4,343,356 teaches that the electric discharge device may be used in any natural fluid within the well casing, including water, brine, oil, solvents, acids, or other chemicals adapted to attack plugging materials. In order to avoid random timing of and an unpredictable path for the discharges, this patent describes initiating the electric discharge with a fine wire bridging the electrodes. Since the wire is vaporized by each discharge, this approach requires a mechanism for replacing the fine wire before each subsequent discharge. Providing a mechanism to feed wire between the electrodes substantially complicates the design of and may reduce the reliability of a well cleaning tool.
Further, experiments conducted by the inventors of the well cleaning method, system and tool described herein have shown that discharging a cleaning tool in an environment that is predominantly oil results in prolonged limited-current discharges that do not produce substantial shock waves and is ineffective for well cleaning. Further, discharging a cleaning tool when oil is present in the discharge head of the tool results in rapid deterioration of insulating surfaces of the tool exposed to the discharges. This deterioration commonly takes the form of erosion or cracking along the insulating surfaces. The cause of the deterioration may be deposition of carbon on the insulating surfaces, which provides a path for the stored energy to discharge across the insulating surface.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is first introduced, and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.